The present application is based on and claims priority from Japanese Patent Applications Hei 8-275677, filed on Oct. 18, 1996 and Hei 9-104370, filed on Apr. 22, 1997, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine, particularly, a stick-like ignition coil installed directly in the plug holes of an internal combustion engine.
2. Description of the Related Art
A high tension terminal of a conventional stick-like ignition coil for an internal combustion engine (hereinafter referred to as the ignition coil) is shown in FIG. 9.
A secondary coil 600 is wound around a secondary spool which has a flange member 603 and a high tension output terminal 602 for providing a high tension voltage generated by the secondary coil 600. A core is inserted in the secondary spool, which usually projects from the secondary coil 600 toward the high tension output terminal, and a spacer portion 604 is formed between the secondary coil 600 and the high tension terminal 602.
If permanent magnets are fixed to an end of the core, the spacer portion 604 becomes longer.
The secondary coil 600 and the high tension output terminal 602 are connected by a lead wire 601 extending from the end of the secondary coil through the spacer portion 604.
However, the stick-like ignition coil has such a small diameter that the low tension primary coil, the core and a portion of the engine block are close to one another. In addition, a surface of the lead wire 601 facing the low tension portion or components is so small that the electric field strength around the lead wire becomes very high.
If a high tension voltage is generated by the secondary coil 600, electric breakdown may arise between the lead wire 601 and the low tension portion, thereby causing failure in supplying high tension voltage to the spark plugs (not shown).
In an ignition coil insulated by insulating resin, a xe2x80x9ctreexe2x80x9d is produced around the lead wire as a result of the electric discharge in the insulating resin and may extend to the low tension components. If the tree bridges the high tension lead wire and the low tension components, the high tension voltage can not be supplied to the spark plugs.
A main object of the present invention is to provide a highly reliable ignition coil which prevents the electric breakdown between high tension portions or components and low tension portions or components.
According to a feature of the present invention, a conductive member is disposed around the portion connecting the secondary coil and the high tension terminal to provide a wide surface area facing low tension components enough to moderate the electric field strength around the connecting section, thereby preventing electric discharge between the connecting section and the low tension components. As a result, a highly reliable ignition coil is provided.
According to another feature of the present invention, the conductive member comprises a cylindrical coil of a wire extended from the secondary coil toward the high tension terminal. Accordingly, a wide surface area of the connecting member is provided in the winding step of the secondary coil without addition of specific steps.
According to another feature of the present invention, the connecting member comprises a cylindrical conductive member which covers the wire connecting the secondary coil and the high tension terminal.
If the adhesive strength of the insulating resin and the primary spool is not enough and the two members are separated from each other due to thermal expansion and contraction, the growing speed of the xe2x80x9ctreeingxe2x80x9d increases in proportion to the length of the separation.
According to another feature of the present invention, the primary spool is made of a material which is bonded to the epoxy resin at a high adhesive strength so that the separation between the insulating resin and the primary spool can be prevented. As a result, even if the treeing grows from a crack and reaches the primary spool, the treeing must bypass the primary spool, so that the time for the treeing to reach the low tension components increases. In other words, time of the electric breakdown is delayed, thereby increasing the life time of the ignition coil.